They combine into Devastator. The Constructicons were the first Special Team with a combined mode, and the only one inherited from the Diaclone line. Uniquely among the Generation 1 combiners, the Constructicons were not interchangeable and could only combine in a very specific way.
"Nobody's driving those things!"
"Right, puny human! We are the Constructicons! We drive ourselves!"
The six members of the constructions are:
Hook, Scrapper, Long Haul, Bonecrusher, Mixmaster, and Scavenger.
The constructicons are brilliant architects and can construct physical wonders at exponential speeds. Sadly, they use their talents primarily for evil. When provoked, the Constructicons can merge to form Devastator, the strongest and most dangerous Decepticon of it's time, next to perhaps only Megatron.